Conventional power assisted steering systems use an engine driven hydraulic pump to provide fluid to a steering mechanism. The steering mechanism has an open center steering control valve that regulates fluid pressure and directs the pressure to the appropriate side of a hydraulic cylinder to provide power assist to steer the vehicle through a linkage system. The cylinder and its associated piston are usually an integral part of the steering mechanism. The engine driven pump of the conventional system has an integral flow control valve that regulates steering to the steering mechanism at an approximately constant rate. The power consumed by this system is proportional to the engine speed and pump outlet pressure. Pumps in such conventional systems are sized to produce the required system flow at engine idle speed. At speeds above idle speed, the integral flow control valve returns excess flow back to the pump. Thus, the energy required to produce the excess flow at a pump outlet pressure is a parasitic loss. A steering control valve is used in a conventional system to control a relationship between valve steering torque and assist pressure. A torsion bar such as a linear torsional spring is used to convert steering torque into valve actuation angle. The relationship of the valve actuation angle and the assist pressure for a constant flow rate is referred to as a boost curve.
Recently, electro-hydraulic power assisted steering systems have been developed. Electro-hydraulic power assisted steering systems have an electric motor that is used to drive a positive displacement pump without an internal flow control valve. System flow is thus controlled by controlling the speed of the motor. In electro-hydraulic power assisted steering systems, the subjective feel of the system is also a function of the boost curve. To reduce total energy, electro-hydraulic power assisted steering systems reduce parasitic losses associated with an engine driven pump and flow control valve. Commonly, system flow is varied in proportion to the turning rate of the steering wheel and the speed of the vehicle. This allows the system flow to be reduced in the non-steering condition. Commonly, flow is also reduced with increased vehicle speed. Lower flows at higher vehicle speeds improve the center feel of the steering, while higher flows at lower speeds and static parking reduce the effort the driver must exert to steer the vehicle. The boost curve changes directly as a function of flow rate.
In designing an electro-hydraulic power assisted steering systems, typically trade-offs must be made. Center feel may be optimized with a high torsion bar stiffness and a boost curve that provides little or no assist at low to moderate steering wheel torques. A low parking effort may be achieved with a low torsion bar stiffness, quick closing boost curve and high flow rates.
It would therefore be desirable to provide an electro-hydraulic power assisted steering system that provides improved steering feel at all the vehicle speeds.